The present invention relates to printing of patterns with extremely high precision on photosensitive surfaces, such as photomasks for semiconductor devices and displays. It also relates to direct writing of semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. Furthermore, it can have applications to other types of precision printing such as security printing. The term printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared (IR) to extreme UV. Of special importance is the ultraviolet range from 370 nm (UV) through deep ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) down to a few nanometers wavelength. EUV is in this application defined as the range from 100 nm and down as far as the radiation is possible to treat as light. A typical wavelength for EUV is 13 nm. IR is defined as 780 nm up to about 20 xcexcm.
In a different sense the invention relates to the art and science of spatial light modulators and projection displays and printers using such modulators. In particular the invention improves the grey-scale properties, the image stability through focus and image uniformity and the data processing for such modulators by application of analog modulation technique. The most important use of the analog modulation is to generate an image in a high-contrast material such as photoresist with an address grid, i.e. the increment by which the position of an edge in the pattern is specified, that is much finer than the grid created by the pixels of the spatial light modulator.
It is known in the current art to build precision pattern generators using projection of micromirror spatial light modulators (SLMs) of the micromirror type (Nelson, U.S. Pat. No. 5,148,157, 1988, Kuck, EP 0610183, 1990). To use an SLM in a pattern generator has a number of advantages compared to the more wide-spread method of using scanning laser spots. The SLM is a massively parallel device and the number of pixels that can be written per second is extremely high. The optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in a laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electrooptic and acoustooptic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device.
In both references cited above the spatial modulator uses only on-off modulation at each pixel. The input data is converted to a pixel map with one bit depth, i.e. with the values 0 and 1 in each pixel. The conversion can be done effectively using graphic processors or custom logic with area fill instructions.
In a previous application by the present inventor Sandstrom (Sandstrom et al., EP 0567076, 1990), the ability to use an intermediate exposure value at the boundary of a pattern element to fine-adjust the position of the element""s edge in the image created by a laser scanner was described.
It is also known in the art to create a grey-scale image, preferably for projection display of video images and for printing, with an SLM by variation of the time a pixel is turned on or by printing the same pixel several times with the pixel turned on a varying number of times. The present invention devices a system for direct grey-scale generation with a spatial light modulator, with a special view to the generation of ultra-precision patterns. Important aspects in the preferred embodiments, are uniformity of the image from pixel to pixel and independence of exact placement of a feature relative to the pixels of the SLM and stability when focus is changed, either with intention or inadvertently.
Specifically there are problems with the prior art to achieve adequate pattern fidelity and dimensional precision in pattern generators using an SLM.
It is therefore an object of the present invention to provide a method for creating a pattern on a workpiece with better pattern fidelity and dimensional precision.
This object is achieved with a method according to the appended claims.
Specifically the method for creating a pattern according to the invention comprises the steps of:
providing a source for emitting electromagnetic radiation in the wavelength range from EUV to IR,
illuminating by said radiation a spatial light modulator (SLM) having multitude of modulating elements (pixels),
projecting an image of the modulator on the workpiece,
moving said workpiece and/or projection system relative to each other,
reading from an information storage device a digital description of the pattern to be written,
extracting from the pattern description a sequence of partial patterns,
converting said partial patterns to modulator signals, and feeding said signals to the modulator,
further coordinating the movement of the workpiece, the feeding of the signals to the modulator and the intensity of the radiation, so that said pattern is stitched together from the partial images created by the sequence of partial patterns,
further exposing said pattern in at least two separate exposures, where at one exposure corrections are applied for errors occurring during another exposure.
By such an additional exposure, several types of errors could be compensated for.